1. Field of the Invention
The present invention relates to a semiconductor device with a wafer level hermetic seal and a method of hermetically sealing a device at the wafer level. More particularly, the present invention relates to a method of hermetically sealing a semiconductor device which is at a wafer level where the device sensitive to high temperatures or affected by a heat cycle can be thermally sealed at a low temperature and is not affected by moisture or particles.
2. Description of the Related Art
Referring to FIGS. 1 and 2, the surfaces of a conventional semiconductor devices, such as MicroElectroMechanical Systems (MEMS), are first micro-machined in order to hermetically seal the devices. The surface micro-machining is a method by which a sacrificial layer is formed, a structure is formed thereon, and then the sacrificial layer is removed, thereby forming a moving structure. In the case where the surface micro-machining is used, structures or semiconductor devices are formed on a wafer (not shown) in a state in which the sacrificial layer (also not shown) is not removed (S201). Then the wafer is sawed in half (S203) and the sacrificial layer is etched to form moving structures (S205). The resulting moving structures must be carefully handled because these structures can be damaged or rendered completely inoperative by a single microscopic particle. Thus, a first anti-stiction film is coated on the wafer (S207) to prevent dust or particles from attaching to the moving structures. This coating also protects the moving structures during the physical moving, handling and testing of the devices. The anti-stiction film is a film that prevents dust or particles from being attached to a surface by lowering the surface energy of the surface.
After the first anti-stiction film coating, each individual device is tested according to a first test (S210). Since the unit cost of a packaging process is high, this first test is necessarily performed to seek high quality devices. The atmosphere where the first test is carried out must be adjusted so that there is almost no moisture therein. By adjusting the atmosphere, high quality devices can be prevented from being transformed into low quality ones during the testing process. The wafer is sawed into individual chips after the testing is completed (S213). Here, a high percentage of the chips become defective due to the particles generated during the sawing process. The equipment used in a general semiconductor process cannot be used during the sawing process, while additional equipment for fabricating devices such as MEMS is required. This results in an increase in the production cost.
Each individual chip 105 is attached to a package 100 by a die adhesive 103 (S215) and electrically connected to the package 100 by a wire-bonding process 118 (S217). Here, the chip is exposed to the outside environment, and moisture or particles can attach to the chip if the anti-stiction film becomes contaminated. Therefore, a cleaning process is performed to remove such particles (S220). However, a second anti-stiction film coating is necessarily required because the first anti-stiction film may also be removed during the cleaning process (S223). Even if the first anti-stiction film is not removed during the cleaning process, surfaces of the chip with prolonged exposure to air will absorb the moisture, making the second anti-stiction film coating necessary. The package containing the chip is now also coated with the second anti-stiction film.
The lid 115 is aligned with the package 100, which is coated with the second anti-stiction film, and hermetically sealed with a seal ring 110 (S225). Here, reference numeral 113 represents a lid frame. By-products, moisture, and particles are generated when the chip 105 is first tested in an unsealed state and then attached to the package 100. These factors can damage the device or make the device completely inoperable. Therefore, a second test is performed after the seal (S227).
As described above, performing the hermetic sealing process in a chip state is costly as well as labor and time intensive. An increase in cost is attributable to additional equipment needed to carefully handle the MEMS devices or chips having moving structures. In addition, maintaining a multiple work environment such as two anti-stiction coating lines and two testing lines is also costly, and labor and time intensive. In particular, the second anti-stiction film is coated on every package before sealing. This process is slow because the area of the package to be coated is large, thereby requiring a large amount of time to coat the anti-stiction film.
The method of hermetically sealing the package 100 and the lid 115 includes welding and glass high temperature splicing. In a typical welding process, the lid frame 113 is attached to the lid 115 while the seal ring 110 is placed in-between the package 100 and the lid 115. Then the lid 115 along with the seal ring 110 is attached to the package 100 by welding. Here, a high-priced ceramic or metal is used to ensure the hermetic state.
A wafer level bonding method includes silicon-silicon fusion bonding, silicon-glass anodic bonding, eutectic bonding using a medium such as Au, and bonding using a glass frit. In these methods, the cleanness of a surface to be sealed is very important and high temperature or pressure is required.
Accordingly, these methods are not appropriate for devices such as MEMS, which use aluminum actuators having a relatively low fusion temperature. In the case of a silicon-glass anodic bonding method, bonding is performed at a relatively low temperature of about 450 C. However, even a temperature of about 450 C is too high for aluminum actuators and high pressure, which may negatively affect the device, is required .